This invention relates to improvements in an electrostatic powder coating booth and, more particularly, to a powder coating booth in which the greater part of powder oversprayed during powder coating is collected on an upper surface of a conveyor belt which forms the bottom surface of the booth; oversprayed powder thereby collected is moved to a position isolated from the bottom of the booth while being kept on the conveyor belt; and the oversprayed powder paint is recovered by a paint recovering device at this position, these improvements increasing the efficiency with which powder paint is used and making it possible for the coating color to be changed rapidly.
Electrostatic powder coating is highly advantageous in terms of coating film performance, prevention of environmental pollution, saving of resources and so forth. Recently, the importance of electrostatic powder coating has been recognized more and more widely. In a conventional electrostatic powder coating process, part of the electrified powder paint ejected from a painting gun does not attach itself to the object to be coated, and the majority of this part of the paint, namely, the oversprayed powder is drawn together with booth exhaust air and is separated and recovered by an exhaust dust collector of a large size. The remaining part of the oversprayed powder attaches to and accumulates on the bottom and the sidewall surfaces of the booth.
In another known system, the bottom surface of the booth is formed by a gas-permeable conveyor belt in order to exhaust air through the conveyor, and oversprayed powder which collects on the surface of the conveyor belt facing the interior of the booth is carried to one end of the conveyor where it is retrieved.
In the type of powder coating booth that is based on the conventional technique which utilizes a booth exhaust dust collector to retrieve oversprayed paint, the size of the booth exhaust dust collector is large and electrified paint powder becomes attached to internal portions of the collector. It is therefore necessary to clean the exhaust dust collector in order to avoid mixing of colors when the paint color is changed, and this cleaning is very much time consuming. This in turn leads to a lengthening of the time during which the coating line work is suspended.
To cope with this problem, in one known method, a plurality of dust collectors are provided for different colors and are changed over during the operation of the coating booth. This arrangement requires an increased investment on equipment, makes the plant layout difficult because the area occupied by the dust collectors for the respective colors is excessively large, and also entails various problems of operational management.
In another method proposed to solve the above-described problem, an auxiliary dust collector is provided separately from the main dust collector for the purpose of facilitating the color changing operation in such a manner that, while one of these collectors is operating, the other is cleaned, and they are changed over during the color changing operation so as to reduce the time during which operations are suspended. In this case, however, the time taken to clean the collector becomes excessively long if the amount of exhaust from the booth is large. Further more, it is difficult to adopt this method in a case where the color-changing frequency is high, even if the capacity of the booth is small.
It is also necessary to clean the inner surfaces of the booth at the time of a color-change, by removing the paint powder that has become attached thereto. This operation is also very time consuming and again reduces the time available for effective operation. There is also a possibility that a quantity of paint that has become attached and accumulated over a long period of time such as to change in quantity may fall and become mixed with the recovered paint, the resulting coating thus becoming defective. In addition, dropping of attached paint powder may cause generation of a gas-powder mixture and the amount of this gas-powder mixture may temporarily exceed the explosion limit. This poses a serious problem in terms of safety.
In the system of using a gas-permeable conveyor (ordinarily a cloth material) which forms the bottom surface of the booth, the speed at which air is transmitted through the conveyor tends to become excessively high which makes it difficult for fine powder to be readily be retrieved. It is also difficult to completely remove the oversprayed paint that collects on the conveyor. There is therefore a need to change the conveyor at the time of a color change, and the time taken to perform the color changing operation is consequently increased. This system is not practical when applied to a color-change booth.